The present invention relates to a wireless receiver set which has the facility to measure the receive frequency for automatic frequency control in a communication system using an angle modulation system. The present invention is preferably applicable to a mobile subscriber set using a narrow band in the UHF band.
The drift of carrier frequency degrades the transmission characteristics considerably. The frequency drift deteriorates the distortion, frequency response characteristics, and/or error rate in the passband and furthermore, that frequency drift increases the undesirable leakage power to adjacent channels. In order to prevent the effect by the frequency drift, one of the following must take place.
(a) The communication system is designed so that the interval of the radio channels is considerably larger than the bandwidth of each channel.
(b) A local oscillator and/or modulator which might cause the frequency drift is sufficiently stable.
(c) The frequency drift is measured, and the drift is compensated for automatically.
Among the above items the first one is not practical, since the wireless channels are limited in number and bandwidth.
The second proposal is also not practical in case of a mobile subscriber set, although it is useful for a fixed base station which may install a highly stable standard oscillator, since a small subscriber set must be simple in structure and can not have a highly stable standard oscillator. A TCXO (Temperature Compensated Crystal Oscillator) has been known for compensating frequency drift of carrier frequency due to temperature change. However, when a TCXO is installed in a mobile subscriber set, the frequency stability is practically up to 1-1.5 ppm (1 ppm=10.sup.-6).
Further, a DTCXO system (Digitally Temperature Compensated Crystal Oscillator) is proposed in IEEE Proc. 37th Frequency control symposium pp 434-441, by T. Uno et al, in which a memory stores the frequency change by temperature change, and a capacitance array is adjusted according to the measured temperature and the content of the memory, has been proposed. However, that DTCXO system can not compensate secular change of an oscillator, although it can provide the stability up to 1 ppm.
The above third proposal is classified into two methods.
(a) AFC (Automatic Frequency Control)
FIG. 1 shows a block diagram of an automatic frequency control system which has been used in a prior FM receiver. An FM signal received by an antenna 1.1 is frequency converted by a mixer 1.3 which receives a local frequency of a local oscillator 1.2. The IF signal (Intermediate Frequency signal) which is the output of the mixer 1.3 is applied to a frequency discriminator 1.5 through an IF amplifier 1.4. The demodulated speech signal is obtained at the output 1.6 of the frequency discriminator 1.5. Since the frequency discriminator 1.5 provides DC voltage which is proportional to frequency error of an input frequency, or the error of the input frequency from the center frequency of the frequency discriminator, that DC voltage is applied to a variable capacitance diode 1.8 through a level converter 1.7 which adjusts the voltage level of that DC voltage to the proper voltage for the operation of the variable capacitance diode 1.8. The variable capacitance diode 1.8 is a component of a resonant circuit of a local oscillator. When the local frequency by the variable frequency oscillator 1.2 drifts, the IF frequency then drifts, and the amount of the drift induces the DC voltage at the output of the discriminator 1.5. That DC voltage is applied to the variable capacitor diode 1.8 so that the DC voltage or the frequency error is decreased.
However, the prior system of FIG. 1 has the disadvantage that the frequency stability is not enough for a narrow band mobile communication, since the error of the center frequency of a frequency discriminator can not be less than 0.5 kHz on the condition of mass production. In other words, the frequency error of the local oscillator can not be less than the error of the center frequency of a discriminator. Therefore, the system of FIG. 1 can not be used in an application which requires very small frequency drift.
(b) Phase lock of a local oscillator to receive frequency
FIG. 2 shows a block diagram of another prior art (Japanese patent publication No. 1544/65), in which a local oscillator is phaselocked to a receive frequency which is sufficiently stable. In FIG. 2, the numeral 2.1 is a reception antenna, 2.2 is a first receiver for communication, 2.3 is a second receiver for frequency control for receiving reference frequency from a base station. The second receiver 2.3 provides the received reference frequency to the phse detector 2.4. The numeral 2.5 is a voltage controlled crystal oscillator, output of which is applied to the phase detector 2.4. The output of the phase detector 2.4 is applied to the oscillator 2.5. Thus, the output frequency of the oscillator 2.5 is locked to the reference frequency from the base station.
The local oscillator for the first receiver 2.2 has another phase detector 2.6, a voltage controlled oscillator 2.7 and a frequency divider 2.8. The output of the crystal oscillator 2.5 is compared with the output of the voltage controlled oscillator 2.7 through the frequency divider 2.8, and the output of the phase detector 2.6 controls the voltage controlled oscillator 2.7 so that the oscillator 2.7 is locked to the crystal oscillator 2.5. Therefore, the accuracy or the stability of the oscillator 2.7 is the same as that of the base station. Since the reference frequency from the base station is supposed to be accurate enough, the local frequency in the configuration of FIG. 2 is also accurate.
However, the system of FIG. 2 has the following two disadvantages. First, a receive side requires not only a first receiver 2.2 for communication, but also a second receiver merely for receiving reference frequency. Thus, the structure of the receiver set must be complicated. Further, the transmission of the reference frequency merely for frequency control of a receiver is not desirable in view of the use of valuable frequency band. Secondly, when the fading is severe, and the receive level is low, it suffers from much phase change, the phase lock loop would not operate correctly, and the regeneration of the reference frequency would be impossible.